In recent years, a biodegradable resin is earnestly investigated in the field of fibers and nonwoven fabrics since a biodegradable resin is decomposed into carbon dioxide and water in a short period of time with microorganisms or the like by burying it in the soil, thereby providing less environmental load as compared to the conventional plastic products.
In particular, a biodegradable nonwoven fabric formed of an aliphatic polyester, such as polylactic acid, polyethylene succinate, polybutylene succinate and poly-caprolactone, has properties as nonwoven fabric that are equivalent to those of versatile synthetic fibers and is being subjected to practical use. Polylactic acid has a relatively high melting point among the biodegradable aliphatic polyesters and has high practical utility, and therefore, polylactic acid is expected to be applied to various purposes.
A nonwoven fabric formed of polylactic acid has biodegradability and is excellent in heat resistance owing to the melting point that is generally higher than other aliphatic polyesters. However, a polylactic acid resin has a small crystallization speed under ordinary spinning conditions although it has good crystallinity. Accordingly, fibers having been spun and cooled still have tackiness among the fibers in the web accumulation process, and fibers constituting the web are bonded to each other to provide a nonwoven fabric that lacks flexibility, which is difficult to apply to such a purpose that the nonwoven fabric is in contact with the human skin.
When a web formed of polylactic acid is thermally bonded or resin-bonded with an adhesive while controlling to prevent the flexibility from being impaired, the resulting nonwoven fabric becomes fluffy or inferior in mechanical strength, thereby failing to provide a nonwoven fabric that can be subjected to practical use.
Such a polylactic acid continuous fiber nonwoven fabric is proposed that the polylactic acid polymer constituting the continuous fibers is a polymer or a blend of polymers each having a melting point of 100 degree Celsius or more selected from poly(L-lactic acid), a copolymer of D-lactic acid and L-lactic acid, and a copolymer of D-lactic acid and a hydrocarboxylic acid, and a copolymer of L-lactic acid and a hydrocarboxylic acid and the continuous fibers constituted by the polylactic acid polymer are partially heat-adhered under pressure (see, for example, Japanese Patent No. 3,434,628). However, the nonwoven fabric is constituted by a single component and thus has hard texture with poor flexibility.
Heat-fusible composite fibers formed of two kinds of polylactic acid polymers having different melting points are proposed (see, for example, JP-A-7-310236). The composite fibers are excellent in adhesion property, but the low melting point component functions as an adhesive component for all the fibers, and therefore, a nonwoven fabric produced from the fibers has hard texture with poor flexibility, as similar to a nonwoven fabric constituted by a single component.